Physical vapor deposition (PVD) is a technology by which thin metallic and ceramic layers may be deposited on a substrate, and includes, for example, sputtering processes. PVD processes can be utilized in, for example, semiconductor device fabrication to form thin films of material over semiconductor substrates. A difficulty in forming thin films over semiconductor substrates can occur in attempting to maintain uniformity of film thickness and composition over an uneven topography. For instance, semiconductor substrates will frequently have trenches and vias formed therein, and a goal of a sputtering process is to provide a thin film having uniform thickness across a surface of the substrate and within the trenches and vias. As semiconductor devices become increasingly smaller, the aspect ratios of the trenches and vias increase, and it becomes increasingly difficult to provide a uniform thin film within the trenches and vias.
Materials of particular importance in the manufacture of sputtering targets are face centered cubic (f.c.c.) metals such as aluminum, copper, gold, and nickel, and their alloys. Three metallurgical properties of sputtering targets that can influence the performance of the targets are material uniformity (the absence of precipitates, voids, inclusions and other defects), the grain size (with finer grains sizes generally being preferred over more coarse grain sizes), and texture (with texture referring to the strength of particular crystallographic orientations; a “weak” texture comprises a substantially random distribution of crystallographic orientations, and a “strong” texture comprises a predominate crystallographic orientation within the crystallographic orientation distribution).
A strongly <220>-oriented texture can provide optimum target performance in terms of deposition rate and film uniformity, and can also lead to good sidewall and step coverage of trenches and vias with high aspect ratios. Targets with strong <220> orientation can be considered to correspond to an optimal case where the angular distribution of sputtered material is concentrated around the direction normal to the target surface. Specifically atom emission of f.c.c materials can occur preferentially, along the <220> close-packed direction. The advantages associated with <220>-oriented texture can be especially pronounced in directional deposition techniques, such as, for example, long throw sputtering, and self-ionized plasma PVD.
In addition to appropriate orientation of texture, small grain size can be an important and desired attribute of a sputtering target. Specifically, small, uniform grain sizes in a sputtering target can enable better-quality thin films to be formed from the target than could be formed from a target having coarser grains. The better-quality aspects that can be achieved with small grains versus coarser grains include, for example, better step coverage of a sputter-coated film over uneven surface topography of an underlying substrate.
It would be desirable to form PVD targets comprising fine grain sizes and strongly <220>-oriented textures (with fine grain size referring to average grain sizes less than about 30 microns, and preferably less than about 1 micron). However, it has proven difficult to obtain such combined properties, and instead average grain sizes are at least 40 microns in targets having <220>-oriented textures. Additionally, it can be difficult to retain <220>-oriented textures during target fabrication. For instance, an axial-oriented <220> texture can be induced in a material by forging cylindrical billets of the material. However, cold or warm forging generally comprises recrystallization annealing of the forged material, and such induces a change in texture from the <220>orientation to the <200> orientation. An effort has been made to avoid such detrimental recrystallization by performing hot forging at a temperature higher than that of static recrystallization of a treated material, and accordingly hot forging has become a widely used method to produce targets with strongly <220>-oriented texture. A difficulty in utilizing hot-forging is that the high processing temperatures and limited straining used in hot forging lead to non-uniform grain sizes significantly larger than 30 microns. Additionally, large second-phase precipitates (>5 microns) can be undesirably induced in a hot-forge-treated target material.
The problems and procedures described above pertain to formation of axial-oriented <220> textures, and similar problems can pertain to formation of planar-oriented <220> textures. It would be desirable to produce planar-oriented <220> textures in addition to the axial-oriented <220> textures, in that planar-oriented <220> textures may be preferred to axial-oriented <220> textures in particular applications, such as, for example, in applications comprising rectangular targets.
As it is difficult to produce PVD targets having axial-oriented or planar-oriented <220> textures with conventional methods, it would be desirable to develop new methods for forming such textures.